An improved apparatus and method for radio direction finding (DF) is set forth, the generic function of which is that of an "automatic bearing indicator" (ABI).
The automatic bearing indicator system and algorithm here presented are directed to improvement of the performance of a class of radio direction finding (DF) systems. The class of DF systems is characterized by an array of antenna elements (the antenna array) to which is connected a beam forming/scanning network (the beam scanner or goniometer), the whole arranged to produce directive beams which can be scanned over some range of azimuth angle, possibly a full 360.degree.. Typically a beam complex comprising two beams which are scanned in synchronism is, or can be, produced. One of the beams in the complex is characterized by a principal response pattern exhibiting two lobes separated by a single, simple null; this beam is commonly called the "difference" beam after the manner of its derivation. It is used, typically, to determine the bearing of the incident radio wave by noting the beam scan angle for which the response to the incident signal is at the bottom (or minimum) of the pattern null. The other beam in the beam complex is characterized by a principal response pattern exhibiting a single lobe, aligned in angle with the null of the difference beam; this beam is commonly called the "sum" beam after the manner of its derivation. It is generally used for purposes ancillary to the DF function. The antenna system is exemplified by, but not limited to, that circularly disposed antenna array (CDAA) and beam scanner (goniometer) frequently called the "Wullenweber" direction finding system.
If the radio signal being received comprises a simple unmodulated carrier, then the scanning process produces beam responses representing the true beam patterns which are characteristic of the antenna array and beam scanner combination. However, most radio signals have intentional modulation impressed on the carrier, the modulation serving to convey the information transmitted by the radio transmitter. The modulation causes the beam responses produced by the scanning process to be modified with the modulation impressed on them so that they no longer simply represent the true beam patterns characteristic of the antenna array and beam scanner. Since the modulation processed carrier is sometimes more or less completely suppressed, the modification of the beam pattern can be quite severe. In all events, the presence of the intentional modulation on the signal tends to obscure or mimic those features of the beam patterns on which determination of the bearing depends.
Additionally and independently, there exist various mechanisms of radio wave propagation which very frequently cause transmission of a radio signal to a DF site by multiple paths. As a result of the multi-path mode of transmission, a multi-component radio wave is incident on the DF antenna array, the various components arrive from somewhat different directions. Typically, the aperture of the antenna array is insufficient (for reasons of economics or site limitations) to allow angular resolution of the several components so that the bearing of each arriving radio wave may be determined separately. The consequence is that the beam scanning process produces beam response patterns which are combinations (superpositions) of the separate responses to each of the individual multi-path components. Even if the signal is a simple unmodulated carrier, responses derived from superposed waves are not true representations of the beam patterns characteristic of the antenna array and beam scanner. The beam pattern distortion so engendered does from time to time degrade or destroy the bearing indication and may cause it to be erroneous; it may, for example, cause the null in the difference beam response to be filled in, to appear at an erroneous location, or to split into two false nulls. The presence of the distortion is characteristic of the existence of the multi-component condition and its nature admits selection of circumstances when the probability of its effect on bearing accuracy is reduced.
Presently, a class of automatic bearing indicator (ABI) devices used in conjunction with Wullenweber/CDAA systems makes use of the split-lobe pattern only. These devices attack the problems engendered by the intentional modulation on the signal by combining the responses produced by several (typically eight or more) stored successive scan cycles to construct a single composite response. As a consequence of the point-by-point averaging of the several accumulated responses, a composite response is produced in which the effects of the intentional carrier modulation are mitigated. However, in the process of mitigating the effects of the intentional modulation, the distortion characteristic of the multi-component condition also is blurred or smeared, significantly degrading its value for indicating the probability that the bearing indication may be either correct or erroneous. The composite response is subjected to a procedure which tests the symmetry of the composite pattern and attempts to determine the existence, location and quality of a null from the composite pattern as a means of determining the bearing. Note that, even when the attempt is successful, such an ABI system can in no case return a bearing value in less than the number of scan cycles required to construct the composite response.